1. Field of the Invention
This invention relates to assemblies of a flow coupler and heat exchanger for coupling first and second flows of electrically conductive liquids such as the primary and intermediate liquid metals of a nuclear reactor, to pump the first flow and to heat the second flow. It is specifically contemplated that this pump/heat exchanger assembly will be incorporated into pool-type nuclear reactors.
2. Reference to Co-pending Applications
Reference is made to the following co-pending, commonly assigned patent applications:
(1) U.S. Ser. No. 822,183, entitled "Electromagnetic Flow Coupler for Regulating Flow Rate/Pressure," filed Jan. 24, 1986 in the names of C. C. Alexion and R. D. Nathenson; and
(2) U.S. Ser. No. 875,151, pending entitled "A Pump/Intermediate Heat Exchanger Assembly For A Liquid Metal Reactor," filed June 17, 1986 in the names of R. D. Nathenson, C. C. Alexion and W. C. Sumpman.
3. Description of the Prior Art
Early in the development of the liquid-metal fast breeder or nuclear reactor (LMFBR), it was recognized that liquid metals could be pumped by electromagnetic (EM) pumps. Such EM pumps offer the advantages of inherent simplicity and the lack of moving parts as compared with conventional, rotating impeller pumps. Such mechanical pumps. Such EM pumps offer the advantages of inherent simplicity and the lack of moving parts as compared with conventional, rotating impeller pumps. Such mechanical pumps have inherent problems associated with vibration or thermal distortion in areas of closely toleranced moving parts, such as bearings or seals. Furthermore, cavitation problems associated with a rotating impeller of mechanical pumps do not exist in an EM pump.
One such EM pump, known as a flow coupler, is particularly adapted to pump the primary flow of liquid metal to be heated by a core of the nuclear reactor. Such flow couplers transfer the internal energy of an intermediate flow of liquid metal to the primary flow, driving or pumping the primary flow.
Early examples of such flow couplers are described in U.S. Pat. No. 2,715,190 of Brill and UK Pat. No. 745,460 of Pulley. In a typical flow coupler, a driven liquid metal in the intermediate flow is directed through a generator duct of the flow coupler. Adjacent to the generator duct is a pump duct, through which flows the primary flow. The intermediate and primary flows of liquid metal within the generator and pump ducts are exposed to a common magnetic field. Passage of the first flow through the common magnetic field generates a relatively low voltage, which produces a large current in the generator duct, which is applied to the pump duct by a short, low resistance electrode disposed between the generator and pump ducts and by return conductors disposed on either side of the ducts. Interaction of the resulting high current in the pump duct with the common magnetic field drives the primary flow in the pump duct. In this manner, the intermediate flow of the liquid metal in the generator duct is "coupled" to the primary flow of the liquid metal in the pump duct. The use of such flow couplers in LMFBR systems is described in "Sodium Electrotechnology at the Risley Nuclear Power Development Laboratories", by D. F. Davidson et al., NUCLEAR ENERGY, 1981, Volume 20, February, no. 1, pp. 79-90. U.S. Pat. No. 4,469,471 of A. R. Keeton, et al. describes an improved embodiment of such a flow coupler.
In U.S. Pat. No. 4,412,785 of W. G. Roman, there is described a flow coupler/heat exchanger assembly for use with a nuclear reactor. The assembly forms an annular region between inner and outer shells. A plurality of tube sets is disposed within the annular region, with relatively large spaces between adjacent tube sets. A magnetic field is established in a radial direction through the annular region. A first conductive fluid, e.g. the intermediate liquid metal, is pumped through the spaces between the tube sets by an enlarged intermediate pump. A second conductive fluid, e.g. the primary liquid metal, is introduced into the tube sets. The radial magnetic flux couples the flow of intermediate liquid metal with the flow of primary liquid metal. The externally pumped flow of the intermediate liquid metal in the spaces between the tube sets through the radial magnetic flux, produces a voltage and a current in a circumferential direction about the annular region. The current passes through the adjacent tubes and the primary liquid metal therein, producing a driving force in the opposite direction, whereby the primary liquid metal is driven or pumped.
In a publication entitled, "High-Efficiency DC Electromagnetic Pumps and Flow Couplers For LMFBRs," EPRI NP-1656, TPS 79-774, Final Report, January 1981, by I. R. McNab and C. C. Alexion, there is described an integral assembly of a heat exchanger and a flow coupler for a pool-type, LMFBR. A plurality of duct modules is disposed in a circle, with a magnetic field coil disposed between adjacent duct modules. Each duct module includes a pump duct through which the primary liquid metal flows and a generator duct through which the intermediate liquid metal flows in an opposite direction. The magnetic flux generated by the magnetic field coil is directed by an iron circuit to form a circular magnetic field through all of the duct modules. In one embodiment, the intermediate liquid metal is introduced into a centrally disposed inlet and directed downwardly to be introduced to an intermediate heat exchanger comprised of a plurality of vertically oriented tubes. The intermediate liquid metal is then directed upward and about these tubes, before being introduced into each of the generator ducts. The primary flow of liquid metal is directed downwardly through the pump ducts, exiting the pump ducts and being introduced into the tubes of the intermediate heat exchanger, flowing downwardly therethrough, before being discharged and recirculated to the nuclear core. It is contemplated that the flow coupler may be located beneath such an intermediate heat exchanger. In the described embodiment, both of the intermediate and primary flows of liquid metal through the flow coupler, are disposed at the relatively high temperature as appears at the outlet of the nuclear core, e.g. in the range of 900.degree. to 1,000.degree. F. If the temperature of the liquid metal flows could be reduced, the electrical efficiency of the flow coupler could be improved. Further, the mechanical design requirements of a flow coupler operating at reduced temperatures would be less demanding. Further, both of the upper, discharge ends of the vertically oriented tubes of the intermediate heat exchanger and of the lower, input ends of the generator ducts of the flow coupler, are supported by but a single tube sheet or support plate. As a result, the generator ducts displace a number of the intermediate heat exchanger tubes, that would otherwise be supported by the single support plate, thus requiring an assembly of greater diameter or dimension to accommodate a given number of tubes, as require to receive a predetermined flow of the intermediate liquid metal therethrough. It is desired for the greatest efficiency of heat transfer between the intermediate and primary flows of liquid metal, to increase the flow of the intermediate liquid metal, while maintaining or reducing the size of the integral assembly of the heat exchanger and flow coupler.
The above-identified application entitled, "A Pump/Intermediate Heat Exchanger Assembly For A Liquid Metal Reactor," describes an assembly of a flow coupler and an intermediate heat exchanger for a nuclear reactor, wherein the flow coupler is disposed beneath the intermediate heat exchanger and in a co-linear relationship therewith. The primary liquid metal is directed from the reactor core and is introduced into the intermediate heat exchanger flowing down through an array of tubes enclosed in an annular cavity of the intermediate heat exchanger. The intermediate liquid metal is fed into the assembly via a centrally disposed "downcomer" pipe through the intermediate heat exchanger to the flow coupler and, in particular, to a first plenum for distributing the intermediate liquid metal to a plurality of flow couplers or duct modules, each comprised of one or more sets of pump and generator ducts. The intermediate liquid metal exits the plenum being directed up in parallel through the generator ducts of the flow coupler modules. The intermediate liquid metal exiting the pump ducts is collected in the second plenum before being introduced into the annular cavity to be heated by the primary liquid metal flowing downwardly through the tubes. After being heated, the intermediate liquid metal is discharged and directed to a steam generator. The cooled, primary liquid metal is discharged from the tubes into a third plenum, before it is directed downwardly in parallel through the plurality of generator ducts, whereby the cooled, intermediate liquid metal is directed at relatively high pressure, i.e. pumped, into a large plenum at the bottom of the nuclear reactor for return to the reactor core.
The above described flow coupler/intermediate heat exchanger assembly employs three plenums for collecting and redirecting either the primary or intermediate liquid metal. Such plenums typically require relatively thick, heavy pressure plates, which account for nearly 40% of the weight of the flow coupler/intermediate heat exchanger assembly. Such heavy, thick pressure plates were used in order to withstand the relatively high pressure differences between the primary and intermediately fluids, as well as to withstand the relatively fast temperature changes as may occur within these assemblies.
Further, the flow coupler/intermediate heat exchanger assembly to be described, is to be incorporated into a pool-type nuclear reactor, wherein it is disposed within a pool of the primary liquid metal, e.g. sodium. When so immersed, it is often desirable that the primary sodium exiting the intermediate heat exchanger be returned directly to the pool to improve mixing within the pool and ensure uniform heat exchanger performance. Further, there are advantages to withdrawing directly the primary liquid metal from the pool and injecting it into the flow coupler.